The present invention relates to converting pulverulent raw materials to a liquefied state as a first step in a melting process. The invention is particularly applicable to melting glass, including flat glass, container glass, fiber glass, and sodium silicate glass. But the invention is applicable to other processes that involve thermally converting a generally flowable, essentially solid state feed material to a molten fluid. These other processes may include metallurgical smelting type operations and fusing of single or multiple component ceramics, metals, or other materials. More particularly, this invention is an improvement in the invention disclosed in U.S. Pat. No. 4,381,934 and U.S. patent application Ser. No. 481,970 filed Apr. 4, 1983, both by G. E. Kunkle and J. M. Matesa.
Continuous glass melting processes conventionally entail depositing pulverulent batch materials onto a pool of molten glass maintained within a tank type melting furnace and applying thermal energy until the pulverulent materials are melted into the pool of molten glass.
The conventional tank type glass melting furnace possesses a number of deficiencies. A basic deficiency is that several operations, not all of which are compatible with one another, are carried out simultaneously within the same chamber. Thus, the melter chamber of a conventional furnace is expected to liquefy the glass batch, to dissolve grains of the batch, to homogenize the melt, and to refine the glass by freeing it of gaseous inclusions. Because these various operations are taking place simultaneously within the melter, and because different components of the glass batch possess different melting temperatures, it is not surprising that inhomogeneities exist from point to point within the melter.
In order to combat these inhomogeneities, a melting tank conventionally contains a relatively large volume of molten glass so as to provide sufficient residence time for currents in the molten glass to effect some degree of homogenization before the glass is discharged to a forming operation. These recirculating flows in a tank type melter result in inefficient use of thermal energy, and maintaining the large volume of molten glass itself presents difficulties, including the need to heat such a large chamber and the need to construct and maintain such a large chamber made of costly and, in some cases, difficult to obtain refractory materials. Moreover, erosion of the refractories introduces contaminants into the glass and requires rebuilding of the melter in a matter of a few years. Additionally, it is known that some components of the batch such as limestone, tend to melt out earlier than the sand and sink into the melt as globules, whereas higher melting temperature components, such as silica, tend to form a residual unmelted scum on the surface of the melt. This segregation of batch components further aggravates the problem of inhomogeneities.
Recent findings have indicated that a major rate limiting step of the melting process is the rate at which partly melted liquefied batch runs off the batch pile to expose underlying portions of the batch to the heat of the furnace. The conventional practice of floating a layer of batch on a pool of molten glass is not particularly conducive to aiding the runoff rate, due in part to the fact that the batch is partially immersed in the molten glass. It has also been found that radiant energy is considerably more effective at inducing runoff than is convective heat from the pool of molten glass, but in a conventional melter, only one side of the batch is exposed to overhead radiant heat sources. Similarly, conventional overhead radiant heating is inefficient in that only a portion of its radiant energy is directed downwardly towards the material being melted. Not only is considerable energy wasted through the superstructure of the furnace, but the resulting thermal degradation of the refractory roof components constitutes a major constraint on the operation of many glass melting furnaces. Furthermore, attempting to heat a relatively deep recirculating mass of glass from above inherently produces thermal inhomogeneities which can carry over into the forming process and affect the quality of the glass products being produced.
Many proposals have been made for overcoming some of the problems of the conventional tank type glass melting furnace, but none has found significant acceptance since each proposal has major difficulties in its implementation. It has been proposed, for example, that glass batch be liquefied on a ramp-like structure down which the liquid would flow into a melting tank (e.g., U.S. Pat. Nos. 296,227; 708,309; 2,593,197; 4,062,667; and 4,110,097). The intense heat and severely corrosive conditions to which such a ramp would be subjected has rendered such an approach impractical since available materials have an unreasonably short life in such an application. In some cases, it is suggested that such a ramp be cooled in order to extend its life, but cooling would extract a substantial amount of heat from the melting process and would diminish the thermal efficiency of the process. Also, the relatively large area of contact between the ramp and each unit volume of glass throughput would be a concern with regard to the amount of contaminants that may be picked up by the glass. Furthermore, in the ramp approach, heat transfer from a radiant source to the melting batch materials is in one direction only.
A variation on a ramp type melter is shown in U.S. Pat. No. 2,451,582 where glass batch materials are dispersed in a flame and land on an inclined ramp. As in other such arrangements, the ramp in the patented arrangement would suffer from severe erosion and glass contamination.
The prior art has also suggested melting glass in rotating vessels where the melting material would be spread in a thin layer on the interior surface of the vessel and would, more or less, surround the heat source (e.g., U.S. Pat. Nos. 1,889,509; 1,889,511; 2,006,947; 2,007,755; 4,061,487; and 4,185,984). As in the ramp proposals, the prior art rotary melters possess a severe materials durability problem and an undesirably large surface contact area per unit volume of glass throughput. In those embodiments where the rotating vessel is insulated, the severe conditions at the glass contact surface would indicate a short life for even the most costly refractory materials and a substantial contamination of the glass throughput. In those embodiments where the vessel is cooled on the exterior surface, heat transfer through the vessel would subtract substantial amounts of thermal energy from the melting process, which would adversely affect the efficiency of the process. In a rotary melter arrangement shown in U.S. Pat. No. 2,834,157 coolers are interposed between the melting material and the refractory vessel in order to preserve the refractories, and it is apparent that great thermal losses would be experienced in such an arrangement. In cyclone type melters, as shown in U.S. Pat. Nos. 3,077,094 and 3,510,289, rotary motion is imparted to the glass batch materials by gaseous means as the vessel remains stationary, but the cyclone arrangements possess all the disadvantages of the rotary melters noted above.
Some prior art processes conserve thermal energy and avoid refractory contact by melting from the interior of a mass of glass batch outwardly, including U.S. Pat. Nos. 1,082,195; 1,621,446; 3,109,045; 3,151,964; 3,328,149; and 3,689,679. Each of these proposals requires the use of electric heating and the initial liquefaction of the batch materials depends upon convective or conductive heating through the mass of previously melted glass. This is disadvantageous because radiant heating has been found to be more effective for the initial liquefaction step. Additionally, only the last two patents listed disclose continuous melting processes. In a similar arrangement disclosed in U.S. Pat. No. 3,637,365, one embodiment is disclosed wherein a combustion heat source may be employed to melt a preformed mass of glass batch from the center outwardly, but it, too, is a batchwise process and requires the melting to be terminated before the mass of glass batch is melted through.
The use of an electric plasma heat source (variously termed plasma arc, plasma jet, plasma torch) has long been recognized as desirable for melting applications because of the extremely high temperatures that can be attained. A major difficulty is providing a vessel that can withstand such high temperatures. Cooling of the melting vessel has been employed to reduce erosion of the vessel, but the resulting extraction of heat significantly reduces the thermal efficiency of the overall process. Therefore, plasma melting has been considered impractical for large scale commercial glass melting operations and the like.
An electric arc source can also generate extremely high temperatures, but a plasma heat source has advantages thereover in the type of melting process to which the present invention pertains. A cumbersome feature of an electric arc is that at least two electrodes must extend into the melting zone itself so that radiation from the arc directly impinges upon the material being melted. With a plasma torch, however, the plasma may be initiated outside the main melting zone and transported into the melting zone by means of the gas stream. Also, consumption of arc electrodes produces CO and/or CO.sub.2 and can cause undesirable reduction reactions in the glass.